Nuclear-Magnetic-Resonance Studies of the Semiconductor-to-Metal Transition in Chlorine-Doped Cadmium Sulfide

Abstract

Spin-lattice relaxation times ($T_1$) and Knight shifts were measured for ${\mathrm{Cd}}^{113}$ nuclei in 12 CdS crystals doped with various amounts of chlorine. Hall coefficients were measured in order to estimate conduction-electron concentrations. Data were obtained for all samples at 300 °K and for some highly doped samples at 77, 4.2, and 2.13 °K. Metallic properties were observed in all samples having electron concentrations $n>\mathrm{}2\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$. At 300 °K, we find $\frac{1}{T_1}\propto n$ for nonmetallic samples and $\frac{1}{T_1}\propto n^{\frac{2}{3}}$ when samples are metallic. The latter proportionality continues to hold at lower temperatures. The dependence of $T_1$ on $n$ becomes increasingly less pronounced at lower temperatures in the nonmetallic samples indicating that the nuclear relaxation becomes at least partially dependent on mechanisms other than conduction electrons, such as spin-diffusion coupling to paramagnetic impurity sites. In the metallic samples, the Knight shift $K\propto n^{\frac{1}{3}}$ and the Korringa product is a constant: $T_{1}T{K}^2=3.3\mathrm{}\times {10}^{-6\mathrm{}}$ sec °K. Both the Knight shift and Korringa product decrease sharply for $n<\mathrm{}2\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Our analysis shows that the Mott transition (formation of an impurity conduction band or transition to "free" conduction) occurs in a region $5\times {10}^{17}<n<\mathrm{}1.6\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and that the impurity conduction band and the CdS conduction band become merged (i.e., the Fermi level crosses into the CdS conduction band) in a region $1.6\times {10}^{18}<n<\mathrm{}2.4\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$.